This invention relates to a method of controlling a speed reduction ratio in a continuously variable speed transmission, and more particularly to a method of controlling a speed reduction ratio while reducing vehicle speed with engine brake applied.
It is well known that the vehicle is generally decelerated utilizing engine brake by releasing the accelerator pedal to make accelerator opening (the depressed amount of the accelerator pedal or the engine throttle opening actuated by the accelerator pedal connected thereto) zero.
That is also the case with vehicles provided with a continuously variable speed transmission. However, since a continuously variable speed transmission can continuously control its speed reduction ratio, and a speed reduction ratio, if controlled, can control the magnitude of engine brake force as well, how to control a speed reduction ratio in vehicle deceleration has been of particular concern from the standpoint of driver comfort and so forth.
Such being the case, heretofore, for example, when reducing vehicle speed with engine brake applied, there have been utilized the methods of controlling speed reduction ratio: a speed reduction ratio is reduced automatically in accordance with the vehicle speed, or as disclosed in the Japanese Patent laid-open Publication No. 60(1985)-95263, the speed reduction ratio is so controlled that the engine brake force is varied based on the vehicle speed.
Such speed reduction ratio control methods as described above, however, do not take into account the running road condition and so forth. For this reason, when an engine brake is applied even at an identical vehicle speed to reduce vehicle speed, the effect of the engine brake may be different depending on the road condition, e.g., a flat road, an ascending road or a descending road, thus resulting in an uncomfortable feeling for the driver during decelerating running with the engine brake. In general it maybe said that the engine brake tends to be too strong for ascending, and too weak for descending.